Methods and Problems of Drilling Implantation in Rural Water Supply in the Fractured Basement Area in Chad: Case of Ouaddaï Geographic

Massing Oursingbé, Zhonghua Tang, Yonoudjoum M. Médard

Abstract

This study focuses on the Ouaddaï Geographic Region, located on the eastern edge of the Chad basin, along the Sudanese border of Darfur where fracturing is well developed. It aims to highlight the factors affecting the productivity of hydraulic structures by the characterization of fracture networks in crystalline and metamorphic rocks using stereotype resulting from the interpretation of satellite images Landsat-7 ETM + and exogenous data collected in Directorate of the Hydraulics Field Knowledge of Ministry of Water. All techniques resulted in the enhancement of structural and linear elements content in the raw images, enabling better geological cartography. The lineaments map obtained after treatment is very dense and consists of lineaments (discontinuities images) in various sizes. The analysis of these fracture networks has enabled us to highlight the presence of certain tectonic directions N40 to 60° E and N110 to 130° E potentially more favorable. The appearance tectonics is a fundamental criterion for the establishment of hydraulic work. However, satellite images study in comparison with aerial photography makes it possible to know what scale is imperative to make study for a project of rural water supply. The statistic of the various data shows a high flow rate of dry borehole (approximately 60%) and nearly 20% of works have a flow rate ranging between 0.5-1.5 m3/s. For this propose, many dry boreholes are mostly localized on anomalies associated with fault zones very wide and very marked and conductive. These zones often correspond to shearing zones. On this kind of anomaly, the drillings should be established on geophysical gradients, but not in the center of anomaly which could be systematically plugged. It may also include the cemented areas which the electrical tomography in pole dipole directs and inverse can refine the establishment. In all this analysis, we have made from some recommendations in order to allow raising some ambiguities relating to borehole establishment in fractured basement: carry out the drilling in the border of the large and conductive geophysical anomalies. Indeed; carry out the borehole just in the inselbergs border to verify certain hydrogeological assumptions which consist in saying that border of these massive outcrops is aquifer; and carry out the drilling in hydraulic border upstream of dyke, acid, basic and quartz vein. All these considerations are essential to increase significantly the success rate of the drillings establishment in the basement areas.

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